Attitude Reference for Tieback/Overlap Processing

ABSTRACT

A method for calculating orientation changes within a borehole using a gyro sensor to detect angular deflection rates in an attitude reference interval as a MWD system is moved from a first to a second location within the borehole. A compass shot may be taken at one or more of the first and second locations using a magnetometer and accelerometer or gyro sensor and accelerometer. Tieback and/or overlap processing may be applied to increase the accuracy of measured orientation within the borehole. Additionally, tieback and/or overlap processing may be applied to adjust sensor model parameters in response to discrepancies between calculated and measured locations. Iterated calculations of orientation change between subsequent intervals may allow MWD orientation to be computed for an entire drilling operation using only a single compass shot.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims priorityfrom U.S. provisional application No. 61/713,164, filed Oct. 12, 2012.

TECHNICAL FIELD/FIELD OF THE DISCLOSURE

The present disclosure relates to surveys in a measurement whiledrilling (MWD) system, and more particularly to processing methods fordata collected by MWD sensors.

BACKGROUND OF THE DISCLOSURE

Accurately and precisely determining the position and orientation of adrilling assembly during drilling operations is desirable, particularlywhen drilling deviated wells. Traditionally, a combination of sensors isused to measure downhole trajectory and subterranean conditions. Datacollected in this fashion is usually transmitted to the surface viaMWD-telemetry known in the art so as to communicate this trajectoryinformation to the surface. Many factors may combine to unpredictablyinfluence the trajectory of a drilled borehole. Accurate determinationof the borehole trajectory is necessary to determine the position of theborehole and to guide the borehole to its geological objective as wellas avoiding collisions with underground objects, geological features,wells, or zones. In other cases, it is desired to intercept undergroundobjects, geological features, wells, or zones.

In some instances, surveying of a borehole using conventional methodsinvolves the periodic measurement of the Earth's magnetic andgravitational fields to determine the azimuth and inclination of theborehole at the bottom hole assembly. Historically, this determinationhas been made while the bottom hole assembly is stationary.Consequently, the along-hole depth or borehole distance between discretesurvey stations is generally from 30 to 60 to 90 feet or more,corresponding to the length of joints or stands of drillpipe added atthe surface. Error accumulated between multiple survey stations causedby, for example, the presence of physical factors or anomalies may skewmeasurement accuracy. For example, MWD systems which rely onmagnetometers may be influenced by magnetic interference both on and offthe drill string. Additionally, gyrocompasses naturally tend to loseaccuracy at higher inclinations which may reduce overall survey accuracyin gyrocompass MWD systems.

SUMMARY

The present disclosure provides for a method for computing theorientation of a MWD system in a borehole. The method may includeproviding a MWD system, the MWD system including multiple sensors, thedata collected by the sensors interpreted by sensor models havingadjustable sensor model parameters, the MWD system having anorientation, the orientation including azimuth and inclination. Themethod may further includes positioning the MWD system on a drillstring. The method may further include positioning the drill string at adepth within a borehole drilled from a surface location, the depthmeasured from the location of the MWD system to the surface location,the direction of the borehole generally being represented by theorientation of the MWD system; moving, by a motion of the drill string,the location of the MWD system within the borehole from a first depth toa second depth; sensing changes in the orientation of the MWD systemusing one or more sensors of the multiple sensors as the MWD system ismoved from the first depth to the second depth; and calculating thechange in orientation of the MWD system between the first and seconddepths.

The present disclosure also provides for a method for computing theorientation of a MWD system in a borehole. The method may includeproviding a MWD system, the MWD system including multiple sensors, thedata collected by the sensors interpreted by sensor models havingadjustable sensor model parameters, the MWD system having anorientation, the orientation including azimuth and inclination. Themethod may further include positioning the MWD system on a drill string.The method may further include positioning the drill string at an upperdepth within a borehole drilled from a surface location into an earthenformation, the depth measured from the location of the MWD system to thesurface location, the direction of the borehole in which the MWD systemgenerally being represented by the orientation of the MWD system; takinga compass shot using two or more sensors of the multiple sensors at theupper depth to determine the orientation of the MWD system at the upperdepth; drilling deeper into the earthen formation, the MWD system thusmoved to a lower depth; moving, by a motion of the drill string, the MWDsystem within the borehole either from the upper depth to the lowerdepth or from the lower depth to the upper depth; sensing changes in theorientation of the MWD system using one or more sensors of the multiplesensors as the MWD system is moved; calculating the change inorientation of the MWD system between the upper and lower depths; andcomputing the orientation of the MWD system at the lower depth using thecalculated change in orientation between the upper and lower depths andthe orientation of the MWD system at the upper depth.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 depicts a drilling operation including a MWD system consistentwith embodiments of the present disclosure.

FIG. 2 depicts a block diagram of the sensors of a MWD system consistentwith embodiments of the present disclosure.

FIG. 3 depicts an alternate block diagram of the sensors of a MWD systemconsistent with embodiments of the present disclosure.

FIG. 4 depicts a survey operation of a MWD system consistent withembodiments of the present disclosure.

FIG. 5 depicts a flow chart of an exemplary survey operation consistentwith embodiments of the present disclosure.

FIG. 6 depicts a flow chart of an exemplary iterative survey operationconsistent with embodiments of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

FIG. 1 depicts the drilling of a deviated borehole with a drill stringcarrying a measurement while drilling (MWD) system. More particularly,drilling rig 10 at surface 15 is shown drilling borehole 20. Drillstring 101 is made up of numerous sections of pipe and includes bottomhole assembly 103 and drill bit 105. As understood in the art, thesections of pipe are threadedly connected and are connected to the topof drill string 101 at drilling rig 10 as borehole 20 is drilled toincrease the length of drill string 101. The pipe sections are oftenadded as two or three pre-connected tubular sections known as a pipestand. In an exemplary case, pipe sections may be approximately 30 feetin length, and pipe stands may be between 60 and 90 feet in length.

MWD system 107 may be included in drill string 101. In some embodiments,such as that depicted in FIG. 1, MWD system 107 may be located as a partof bottom hole assembly 103. In other embodiments, MWD system 107 may bepositioned in a different location along drill string 101.

As depicted in FIG. 2 an exemplary MWD system 207 is depicted as a blockdiagram. MWD system 207 may include accelerometers 209 x, 209 y, 209 zeach positioned to measure acceleration in mutually orthogonal axes (x,y, z). In particular, the outputs of accelerometers 209 x, 209 y, and209 z may be used to determine the Earth's gravitational force vectorrelative to MWD system 207. In some embodiments, one of the axes (heredepicted as z) may be aligned with MWD system 207.

MWD system 207 may also include magnetometers 211 x, 211 y, 211 z eachpositioned to measure magnetic flux in the x, y, and z axisrespectively. The outputs of magnetometers 211 x, 211 y, 211 z may beused to determine the Earth's magnetic field vector relative to MWDsystem 207.

Additionally, MWD system 207 may include gyro sensor 213. Gyro sensor213 may, as depicted in FIG. 2, be a microelectromechanical system(MEMS). Gyro sensor 213 may be positioned to detect angular changes inthe x, y, and z axes. Although not depicted, MWD system 207 may alsoinclude a data processing system. Additionally, MWD system 207 mayinclude a communications apparatus for communicating with sensorslocated elsewhere on drill string 101 and for communicating with surface15.

As depicted in FIG. 3, an alternate MWD system 307 may include only twoaccelerometers 309 y, 309 c. In such a configuration, y-axisaccelerometer 309 y may be positioned to measure acceleration in they-axis. Cant-accelerometer 309 c may be positioned to measureacceleration in a direction in the x-z plane. In some embodiments, cantaccelerometer 309 c may measure acceleration in a direction oriented at45° between the x and the z axis in the x-z plane. MWD system 307 mayalso include two single-axis gyro sensors 313 x, 313 z aligned tomeasure angular changes in the x and z axes respectively. In some suchembodiments, accelerometers 309 y and 309 c and single-axis gyros 313 xand 313 z may be mounted on a gimbal platform rotatable around thez-axis of the system. When MWD system 307 is still, a compass shot maybe performed by rotating the platform to multiple positions allowing,for example, removal of sensor biases as well as determination of theattitude of the tool as understood in the art. This exemplary embodimentadditionally may enable mechanization of space stabilized continuoussurvey modes utilizing the z-axis gyro and/or y-axis accelerometer forstabilization as understood in the art. In some embodiments, acontinuous reading of the attitude of MWD system 307 may be obtainedwith respect to the gravity vector determined by accelerometers 309 yand/or 309 c and with respect to true north determined by single-axisgyro 313 x. Attitude, as understood in the art, refers to theorientation of MWD system 307 with respect to both gravity (inclination)and either magnetic north or true north (azimuth). Inclination refers tothe vertical declination between well bore 20 and a horizontal plane.The horizontal plane may be nominally defined as a plane normal to aradius of the Earth. Azimuth, as understood in the art, may be definedas the angle of well bore 20 relative to due north as projected on thehorizontal plane.

Although depicted separately, one having ordinary skill in the art withthe benefit of this disclosure will understand that MWD systems 207 and307 may of course be used in a single drill string 101 within the scopeof this disclosure. Furthermore, other configurations of accelerometers,magnetometers, and gyro sensors as known in the art may be used withoutdeviating from the scope of this disclosure. The describedconfigurations are for reference alone and are not intended to limit thescope of this disclosure.

In some embodiments, by combining the readings from two or more sets ofsensors measuring different quantities, usually when the sensors arestationary, an absolute orientation for drill string 101 can becalculated. Such an operation is known as a compass shot. For example,with reference to FIG. 2, by combining the accelerometer readings ofaccelerometers 209 x, 209 y, 209 z of the Earth's gravitational fieldwith the magnetometer readings of magnetometers 211 x, 211 y, 211 z ofthe Earth's magnetic fields, a so-called magnetic azimuth may bedetermined. Instead, with reference to FIG. 3, by combining theaccelerometer readings of accelerometers 309 y, 309 c of the Earth'sgravitational field with the gyro readings of single-axis gyro sensors313 x, 313 z of the Earth's rotation rate, a so-called geographicazimuth can be determined.

As understood in the art, the raw sensor data for each sensor isinterpreted according to a sensor-specific model which takes intoaccount certain sensor model parameters. Sensor model parameters may bepre-determined before MWD system 107 is used in a drilling operation.Consequently, the preset values may not reflect the actual operation ofthe sensors during the drilling operation. The sensor model parametersmay be updated from time to time during the operation of the tool inwhich they are positioned. The determination of sensor model parametersmay depend, in part, on comparing the behavior of the sensors positionedin MWD system 107 at different positions implicit to differentorientations and/or sensor states during a drilling operation. Over thecourse of the drilling operation, invariants (such as the Earth gravityvector, magnetic flux (North), and magnetic flux (vertical) for amagnetic azimuth calculation as previously discussed) are measured, andmay be reconciled to their true values by, for example, adapting thesensor model parameters in response to observed errors across multipleobservations. In some embodiments, for instance, parameter updates maytake the form of a weighted average combination of the measurementobservations. In certain embodiments, sensor-specific models may includewithout limitation bias and scale factor corrections. For example, asensor-specific model for a gyro sensor may include, but is not limitedto, such specifications as: IEEE Standard Specification Format Guide andTest Procedure for Coriolis vibratory Gyros, IEEE Standard 1431, 2004;IEEE Standard Specification Format Guide and Test Procedure for SingleAxis Interferometric Fiber Optic Gyros, IEEE Standard 952, 1997; IEEESpecification Format for Single-Degree-of-Freedom Spring-Restrained RateGyros, IEEE Standard 262, 1969 (Rev. 2010).

During drilling operations, as illustrated in FIG. 4, MWD system 407,positioned on a drill string (not shown), is moved from a first point P1to a second point P2 in borehole 20. P1 may be either above or below P2,that is closer or further from the surface. In some embodiments, P1 andP2 may correspond to the locations of MWD system 407 when a given pipestand is fully inserted and fully retracted from borehole 20respectively.

In an exemplary attitude reference computation procedure 500, withrespect to FIGS. 4, 5, the drill string is stopped, and MWD system 407is held stationary at position P1 (501). In some embodiments, P1 may bethe position corresponding with MWD system 407 within borehole 20 at thecompletion of the drilling of a pipe stand. At this point, a compassshot may or may not be taken (503) using, for example, accelerometer andgyro sensors or accelerometer and magnetometers depending on theconfiguration of MWD system 407.

MWD system 407 is then placed into attitude reference mode (505). Whilein attitude reference mode, MWD system 407 is configured to makecontinuous attitude reference measurements. In some embodiments, theattitude reference measurements are in the form of angular deflectionrates of MWD system 407 using, for example, gyro sensors. For example,as illustrated in FIG. 5, angular deflection rates as ω_(x), ω_(y),ω_(z) correspond to the orientation change rates output by a three-axisgyro sensor. Other configurations discussed here and otherwise mayinstead be used. For example, in a MWD system 407 utilizing a lasergyro, absolute orientation change may be output. One having ordinaryskill in the art with the benefit of this disclosure will understandthat any configuration of sensors capable of measuring orientationchange rate of MWD system 407 may be utilized within the scope of thisdisclosure. For the sake of clarity, MWD system 407 will be discussed asutilizing a three-axis gyro sensor.

The attitude reference measurements are taken frequently enough tocapture essentially all the relative orientation change taken by MWDsystem 407 as it moves along borehole 20. Thus the orientation changebetween the endpoints of a given motion within borehole 20 can becalculated effectively.

While stationary at P1, gyro sensors in certain embodiments of the MWDsystem 407 may be drift tuned to, for example, remove the bias outputsof the gyro sensors so that angular rates associated with the system'sorientation change may be determined with better accuracy. Given thatthe system is still, these biases may include composites of a puresensor bias and modeled outputs associated with, but not limited to, therotation of the Earth in inertial space and orientation with respect togravity. In some embodiments, these biases may also be used for, forexample, the purposes of gyro quality assessment, model parameterupdate, and/or orientation determination. In a case where, for example,gyro sensors show little or no departure from their anticipated bias atstandstill, the drift tuning operation may be omitted.

MWD system 407 may then be moved, by moving the drill string, toposition P2 within borehole 20 (507). In some embodiments, P2 may be theposition corresponding with MWD system 407 within borehole 20 afterwithdrawing the drill string the length of the last completed pipestand. The traverse of MWD system 407 between P1 and P2 defines attitudereference interval 450. One having ordinary skill in the art with thebenefit of this disclosure will understand that the relative position ofP1 and P2 within borehole 20 is arbitrary as long as the depth of P1 andP2 are known. Such a determination may be relatively simple as thelength of the assembled drill string is known. Thus, P1 may be closer tothe surface (uphole) or farther from the surface (downhole) than P2.

While traversing attitude reference interval 450, MWD system 407continuously measures angular deflection rates ω_(x), ω_(y), ω_(z). WhenMWD system 407 reaches P2, the drill string is stopped, and attitudereference mode is disabled. The orientation of MWD system 407 at thedownhole position (P1 or P2) is then computed based on the orientationof the uphole position (P2 or P1) and the orientation change measured onattitude reference interval 450. This orientation change may becalculated, as understood in the art, as the integral of the measuredangular deflection rates ω_(x), ω_(y), ω_(z).

In some embodiments, the orientation of MWD system 407 within borehole20 may be known at one of P1 or P2 before MWD system 407 traversesattitude reference interval 450. For example, in an example in which P1is the downhole position and corresponds with the position of MWD system407 at the completion of drilling a drill stand, and P2 corresponds withthe position of MWD system 407 at the beginning of drilling the drillstand (alternatively characterized as the completion of the previousdrill stand), the orientation at P2 may be known. The orientation at P2may have been determined by a compass measurement already taken at thislocation or may have been calculated as part of a previous attitudereference computation procedure 500. In other embodiments, a compassmeasurement may be taken once MWD system 407 reaches P2 (509).

Generally speaking, since the orientation at the downhole location isthe unknown, and the orientation at the uphole location is known fromprevious iterations, a compass shot may be taken at only the downholelocation. Depending on whether MWD system 407 moves from an uphole P1 toa downhole P2 (a “pushdown interval”) or a downhole P1 to an uphole P2(a “pullback interval”), only one compass shot (503 or 509) is taken. Inother embodiments, orientation of MWD system 407 is calculated only bymeasurements made during repeated attitude reference intervals 450, andcompass shots are taken at neither P1 nor P2, though the orientation atthe uphole location is known from the previous iterations.

The ultimate reference orientation computed by attitude referencecomputation procedure 500 for the downhole unknown position is thuscalculated as a weighted average of the measured orientation changebetween P1 and P2 and any compass shots. For example, where a compassshot is taken only at the uphole location, the orientation at thedownhole location may be computed as the vector sum of the orientationdetermined by the compass shot and the integral of the angulardeflection rates ω_(x), ω_(y), ω_(z), measured during attitude referencecomputation procedure 500.

In fact, in some embodiments, a single compass shot alone may be usedfor an entire drilling operation. The compass shot may be made near thesurface. The orientation of MWD system 407 as borehole 20 is drilled iscalculated by repeating attitude reference computation procedure 500 foreach pipe stand used during the drilling operation. Thus, theorientation of MWD system 407 is determined as the summation of theorientation changes between the beginning and completion of each pipestand, each orientation change calculated by a separate attitudereference computation procedure 500.

In some embodiments, neither P1 nor P2 of a given attitude referencecomputation procedure 500 may correspond to a location having a knownorientation. For example, multiple iterations of attitude referencecomputation procedure 500 may exist between the position of MWD system407 at the completion and at the beginning of a single pipe stand. Insuch an embodiment, attitude reference interval 450 measurements arerepeated and summed until a P2 with known orientation is reached—i.e. ata position corresponding to the beginning of the pipe stand—and theorientation of MWD system 407 at the farthest downhole location of theseries of attitude reference intervals 450 is calculated as the sum ofthe orientation intervals leading to the known P2.

In some embodiments, given a compass measurement (503 or 509) at P1, P2,or both P1 and P2, multistation processing may be applied to betterdetermine sensor model parameters (513). Such multistation processingmay use data obtained by measurements through attitude referencecomputation procedure 500 and, for example, data from a previousattitude reference computation procedure or data from other sensors.

For example, tieback processing could be applied to adapt sensor modelparameters for gyro sensors in a MWD system. Because compass shots madeusing gyro sensors naturally tends to decrease in accuracy at higherborehole inclinations, adaptations in the sensor model parameters toaccount for this degradation may increase accuracy of subsequent compassshots using the gyro sensor. In such a case, the azimuth calculated bythe gyro sensor model may be used to correct a compass measurement tomatch the azimuth as determined by the attitude reference computationprocedure. Alternatively, the mass unbalance terms of the gyro sensormodel may be shifted to correct the measured azimuth. In some cases, acombination of the two approaches may be used simultaneously. Forsubsequent compass shots, the calculated offsets may be applied to, forexample, increase the accuracy of the compass shot whether or not asubsequent attitude reference computation procedure is carried out. Insome embodiments, data from more than one attitude reference computationprocedure 500 may be combined to, for example, improve the accuracy ofsensor model parameter estimations. In some embodiments, comparison ofthe model parameters calculated from different individual or sets ofattitude reference data may be used to detect sensor damage.

Similarly, tieback processing may be applied to the measurements made bymagnetometers in a MWD system. Magnetic interference downhole, caused,for example, by the natural or artificial presence of ferromagneticmaterials in the formation surrounding the borehole and, indeed, by thedrill string itself, may cause inaccuracy with the proper detection ofthe Earth magnetic field. By comparing the azimuth as determined by acompass shot using the magnetometer with the azimuth as determined bythe attitude reference computation procedure, the on- andoff-drillstring interference may be separated from the detected Earthmagnetic field. Alternatively, one or more compass shots and/or sectionsof attitude reference mode data from the gyro sensors may be comparedsimultaneously to one or more magnetic compass shots to separate the on-and off-drillstring interference from the detected Earth magnetic field.Thus, interference can be taken into account in the magnetometer sensormodel. Alternatively, in some embodiments, only on-drillstringinterference may be separated from the detected magnetic field, allowingfor ranging with proximate wellbores while drilling using themagnetometer.

Similarly, tieback processing may be applied to adjust sensor modelparameters of accelerometers in a MWD system.

In some embodiments, one or more magnetic compass shots may be comparedto one or more gyro sensor compass shots and/or sections of attitudereference mode data to correct parameters of the gyro sensor model. Inthis case, both magnetic compass shots known to be free of on- andoff-drillstring interference and magnetic compass shots corrected usingpreviously determined adjustments to the magnetometer sensor model maybe used to correct gyro sensor model parameters.

In some embodiments, overlap processing may be used to address othersources of inaccuracy. For example, as depicted in FIG. 6, two attitudereference computation procedures 603, 607 are carried out with adrilling operation 605 separating them. The measured orientation of theendpoint of the first attitude reference computation procedure 603 maybe compared with that of the start of the second attitude referencecomputation procedure 607. Since, in theory, both of these positionsshould be the same, any inaccuracies (including, for example centeroffset) may be detected and accounted for in the sensor models. In someembodiments, the matching at the interval boundary may be weightedaccording to, for example, estimates of center offset error against thebest estimate of orientation accuracy that could be made otherwise.

One having ordinary skill in the art with the benefit of this disclosurewill understand that the attitude reference computation procedures 603,607 need not be separated by a drilling procedure, and that drillingprocedure 605 is included simply to illustrate the separation of theattitude reference computation procedures 603, 607. In otherembodiments, multiple attitude reference computation procedures may becarried out across the length of a single pipe stand. In fact, althoughvibrations associated with drilling procedures may introduce noise intocomputation, in some embodiments, attitude reference computationprocedures 603, 607 may be carried out during drilling procedures 601,605. Thus orientation changes may be continuously sampled during thedrilling of the full length of a pipe stand.

Furthermore, one having ordinary skill in the art with the benefit ofthis disclosure will understand that more than one of the previouslydescribed processes may be utilized during the course of drilling asingle well. By using both tieback and overlap processing, additionalaccuracy may be achieved. The weighting of the two processing methodsmay be determined by a predetermined error model taking into account therespective accuracies of the two.

The foregoing outlines features of several embodiments so that a personof ordinary skill in the art may better understand the aspects of thepresent disclosure. Such features may be replaced by any one of numerousequivalent alternatives, only some of which are disclosed herein. One ofordinary skill in the art should appreciate that they may readily usethe present disclosure as a basis for designing or modifying otherprocesses and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein. Oneof ordinary skill in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A method comprising: providing a MWD system, the MWD system includingmultiple sensors, the data collected by the sensors interpreted bysensor models having adjustable sensor model parameters, the MWD systemhaving an orientation, the orientation including azimuth andinclination; positioning the MWD system on a drill string; positioningthe drill string at a depth within a borehole drilled from a surfacelocation, the depth measured from the location of the MWD system to thesurface location, the direction of the borehole generally beingrepresented by the orientation of the MWD system; moving, by a motion ofthe drill string, the location of the MWD system within the boreholefrom a first depth to a second depth; sensing changes in the orientationof the MWD system using one or more sensors of the multiple sensors asthe MWD system is moved from the first depth to the second depth; andcalculating the change in orientation of the MWD system between thefirst and second depths.
 2. The method of claim 1, further comprising:taking a compass shot using two or more sensors of the multiple sensorsat the first depth to determine an estimated orientation of the MWDsystem at the first depth; and computing a computed orientation of theMWD system at the second depth using the calculated change inorientation between the first and second depths and estimatedorientation of the MWD system at the first depth.
 3. The method of claim2, further comprising: moving, by a motion of the drill string, the MWDsystem from the second depth to a third depth; sensing changes in theorientation of the MWD system using one or more sensors of the multiplesensors as the MWD system is moved from the second depth to the thirddepth; and calculating the change in orientation of the MWD systembetween the second and third depths; computing a computed orientation ofthe MWD system at the third depth using the calculated change inorientation between the second and third depths and the computedorientation of the MWD system at the second depth.
 4. The method ofclaim 1, further comprising: taking a compass shot using two or moresensors of the multiple sensors at the first depth to determine anestimated orientation of the MWD system at the first depth; taking acompass shot using two or more sensors of the multiple sensors at thesecond depth to determine an estimated orientation of the MWD system atthe second depth; and computing a computed orientation of the MWD systemat the second depth using the calculated change in orientation betweenthe first and second depths and estimated orientation of the MWD systemat the first depth; comparing the computed orientation of the MWD systemat the second depth to the estimated orientation of the MWD system atthe second depth to calculate an offset error; adjusting one or more ofthe adjustable sensor model parameters of one or more sensors of themultiple sensors in response to the offset error.
 5. The method of claim4, further comprising: drilling to a third depth using the drillingstring; and guiding the drill string during the drilling operation usingthe at least one adjusted sensor model parameter.
 6. The method of claim4, wherein the adjusted sensor model parameter is used to identifysensor damage.
 7. The method of claim 1, wherein at least one sensor ofthe multiple sensors is a gyro sensor, and the sensor model parametersof the sensor model for the gyro sensor comprise azimuth offset and massunbalance terms.
 8. The method of claim 7, further comprising: holdingthe MWD system still; drift tuning the gyro sensor to detect and removebias outputs of the gyro sensor.
 9. The method of claim 8, wherein thedetected bias may be used for at least one of quality assessment, modelparameter update, or orientation determination.
 10. The method of claim1, wherein at least one sensor of the multiple sensors is amagnetometer, and the sensor model parameters of the sensor model forthe magnetometer comprise at least one of natural or artificial magneticanomalies affecting the magnetometer
 11. The method of claim 10, whereinat least one artificial magnetic anomaly comprises a second borehole,and a range between the MWD system and the second borehole is calculatedusing data collected by the magnetometer along the borehole.
 12. Themethod of claim 1, wherein at least one sensor of the multiple sensorsis an accelerometer, and the sensor model parameters of the sensor modelfor the accelerometer comprise an acceleration offset.
 13. The method ofclaim 1, further comprising: taking a compass shot using two or moresensors of the multiple sensors at the second depth to determine anestimated orientation of the MWD system at the second depth; andcomputing a computed orientation of the MWD system at the first depthusing the calculated change in orientation between the first and seconddepths and estimated orientation of the MWD system at the second depth.14. The method of claim 13, further comprising: moving, by a motion ofthe drill string, the MWD system from a third depth to the second depth;sensing changes in the orientation of the MWD system using one or moresensors of the multiple sensors as the MWD system is moved from thethird depth to the second depth; and calculating the change inorientation of the MWD system between the third and second depths; andcomputing a computed orientation of the MWD system at the third depthusing the calculated change in orientation between the third and seconddepths and the computed orientation of the MWD system at the seconddepth.
 15. The method of claim 14, further comprising: taking a compassshot using two or more sensors of the multiple sensors at the thirddepth to determine an estimated orientation of the MWD system at thethird depth; computing a second computed orientation of the MWD systemat the second depth using the calculated change in orientation betweenthe third and second depths and the estimated orientation of the MWDsystem at the third depth; comparing the first computed orientation ofthe MWD system at the second depth to the second computed orientation ofthe MWD system at the second depth to calculate an offset error;adjusting one or more sensor model parameters of one or more sensors ofthe multiple sensors in response to the offset error.
 16. The method ofclaim 15, further comprising: drilling to a fourth depth using thedrilling string; and guiding the drill string during the drillingoperation using the at least one adjusted sensor model parameter. 17.The method of claim 1, further comprising moving, by a motion of thedrill string, the MWD system from a third depth to a proximate depth,the proximate depth defined as the closer of the first and second depthsto the third depth; sensing changes in the orientation of the MWD systemusing one or more sensors of the multiple sensors as the MWD system ismoved from the third depth to the proximate depth; and calculating thechange in orientation of the MWD system between the third and proximatedepths.
 18. The method of claim 17, wherein the orientation at theproximate location is known; and computing a computed orientation of theMWD system at the third depth using the calculated change in orientationof the MWD system between the third and first depths and the knownorientation of the MWD system at the first depth.
 19. The method ofclaim 1, further comprising: taking a compass shot using two or moresensors of the multiple sensors at the first depth to determine anestimated orientation of the MWD system at the first depth; taking acompass shot using two or more sensors of the multiple sensors at thesecond depth to determine an estimated orientation of the MWD system atthe second depth; computing a computed orientation of the MWD system atthe second depth using the calculated change in orientation and theestimated orientation of the MWD system at the first depth; comparingthe estimated and computed orientations of the MWD system at the seconddepth to calculate an offset error; and computing a computed orientationof the MWD system at the first depth using the offset error and theestimated orientation of the MWD system at the first depth.
 20. Themethod of claim 19, further comprising: adjusting one or more sensormodel parameters of one or more sensors of the multiple sensors inresponse to the offset error.
 21. The method of claim 20, furthercomprising: drilling to a third depth using the drilling string; andguiding the drill string during the drilling operation using the atleast one adjusted sensor model parameter.
 22. The method of claim 1,further comprising: taking a compass shot using two or more sensors ofthe multiple sensors between the first depth and the second depth todetermine an estimated transition orientation.
 23. The method of claim1, wherein the moving operation further comprises drilling into anearthen formation.
 24. A method comprising: providing a MWD system, theMWD system including multiple sensors, the data collected by the sensorsinterpreted by sensor models having adjustable sensor model parameters,the MWD system having an orientation, the orientation including azimuthand inclination; positioning the MWD system on a drill string;positioning the drill string at an upper depth within a borehole drilledfrom a surface location into an earthen formation, the depth measuredfrom the location of the MWD system to the surface location, thedirection of the borehole in which the MWD system generally beingrepresented by the orientation of the MWD system; taking a compass shotusing two or more sensors of the multiple sensors at the upper depth todetermine the orientation of the MWD system at the upper depth; drillingdeeper into the earthen formation, the MWD system thus moved to a lowerdepth; moving, by a motion of the drill string, the MWD system withinthe borehole either from the upper depth to the lower depth or from thelower depth to the upper depth; sensing changes in the orientation ofthe MWD system using one or more sensors of the multiple sensors as theMWD system is moved; calculating the change in orientation of the MWDsystem between the upper and lower depths; and computing the orientationof the MWD system at the lower depth using the calculated change inorientation between the upper and lower depths and the orientation ofthe MWD system at the upper depth.
 25. The method of claim 24, whereinthe moving, sensing, and calculating operations are repeated for one ormore subsequent drilling operations, wherein the lower depth of a giveniteration corresponds with the upper depth of a subsequent iteration,and the orientation of the MWD system at the lower depth of the giveniteration corresponds with the orientation of the MWD system at theupper depth for the subsequent iteration.